fix: restore customFileHandlers provider

The removal of `customFileHandlers` turned out to be more disruptive change than anticipated as this provider is still used by several popular plugins: #3619, codymikol/karma-webpack#462, angular/angular-cli#19815. Hence we restore the provider and print a deprecation warning to make upgrading easier. This should give more time for the plugin authors to release new versions and users to adopt these versions.

lib/server.js

@@ -82,6 +82,8 @@ class Server extends KarmaEventEmitter {
       filesPromise: ['factory', createFilesPromise],
       socketServer: ['factory', createSocketIoServer],
       executor: ['factory', Executor.factory],
+      // TODO: Deprecated. Remove in the next major
+      customFileHandlers: ['value', []],
       reporter: ['factory', reporter.createReporters],
       capturedBrowsers: ['factory', BrowserCollection.factory],
       args: ['value', {}],

lib/web-server.js

@@ -16,6 +16,25 @@ const proxyMiddleware = require('./middleware/proxy')
 
 const log = require('./logger').create('web-server')
 
+function createCustomHandler (customFileHandlers, config) {
+  let warningDone = false
+
+  return function (request, response, next) {
+    const handler = customFileHandlers.find((handler) => handler.urlRegex.test(request.url))
+
+    if (customFileHandlers.length > 0 && !warningDone) {
+      warningDone = true
+      log.warn('The `customFileHandlers` is deprecated and will be removed in Karma 7. Please upgrade plugins relying on this provider.')
+    }
+
+    return handler
+      ? handler.handler(request, response, 'fake/static', 'fake/adapter', config.basePath, 'fake/root')
+      : next()
+  }
+}
+
+createCustomHandler.$inject = ['customFileHandlers', 'config']
+
 function createFilesPromise (emitter, fileList) {
   // Set an empty list of files to avoid race issues with
   // file_list_modified not having been emitted yet
@@ -58,6 +77,8 @@ function createWebServer (injector, config) {
   handler.use(injector.invoke(sourceFilesMiddleware.create))
   // TODO(vojta): extract the proxy into a plugin
   handler.use(proxyMiddlewareInstance)
+  // TODO: Deprecated. Remove in the next major
+  handler.use(injector.invoke(createCustomHandler))
 
   if (config.middleware) {
     config.middleware.forEach((middleware) => handler.use(injector.get('middleware:' + middleware)))